OsNRT2.4 encodes a dual-affinity nitrate transporter and functions in nitrate-regulated root growth and nitrate distribution in rice

Plant NRT2 nitrate transporters commonly require a partner protein, NAR2, for transporting nitrate at low concentrations, but their role in plants is not well understood. In this study, we characterized the gene for one of these transporters in the rice genome, OsNRT2.4, in terms of its activity and roles in rice grown in environments with different N supply. In Xenopus oocytes, OsNRT2.4 alone without OsNAR2 co-expression facilitated nitrate uptake showing biphasic kinetics at a wide concentration range, with high- and low-affinity KM values of 0.15 and 4 mM, respectively. OsNRT2.4 did not have nitrate efflux or IAA influx activity. In rice roots, OsNRT2.4 was expressed mainly in the base of lateral root primordia. Knockout of OsNRT2.4 decreased lateral root number and length, and the total N uptake per plant at both 0.25 and 2.5 mM NO3- levels. In the shoots, OsNRT2.4 was expressed mainly in vascular tissues, and its knockout decreased the growth and NO3--N distribution. Knockout of OsNRT2.4, however, did not affect rice growth and N uptake under conditions without N or with only NH4+ supply. We conclude that OsNRT2.4 functions as a dual-affinity nitrate transporter and is required for nitrate-regulated root and shoot growth of rice.